Evaluating the Effect of Iron(III) in the Preparation of a Conductive Porous Composite Using a Biomass Waste-Based Starch Template

In this work, the effect of iron(III) in the preparation of a conductive porous composite using a biomass waste-based starch template was evaluated. Biopolymers are obtained from natural sources, for instance, starch from potato waste, and its conversion into value-added products is highly significant in a circular economy. The biomass starch-based conductive cryogel was polymerized via chemical oxidation of 3,4-ethylenedioxythiophene (EDOT) using iron(III) p-toluenesulfonate as a strategy to functionalize porous biopolymers. Thermal, spectrophotometric, physical, and chemical properties of the starch template, starch/iron(III), and the conductive polymer composites were evaluated. The impedance data of the conductive polymer deposited onto the starch template confirmed that at a longer soaking time, the electrical performance of the composite was improved, slightly modifying its microstructure. The functionalization of porous cryogels and aerogels using polysaccharides as raw materials is of great interest for applications in electronic, environmental, and biological fields.


Introduction
Natural polymers enable the production of innovative materials within the framework of the principles of the circular economy, and it is a key strategy to balance progress and sustainability [1]. The circular economy concept is based on material resource reuse, reducing the generation of waste, relying on efficient green technologies that reduce the use of hazardous chemicals, and promoting growth through innovation [2]. A biomass is a sustainable, green, non-toxic [3], biodegradable [4], and abundant natural resource [5], and a significant portion of lignocellulosic biomass waste is discarded from agricultural activities [6]. For instance, starch is a natural polymer obtained from potato agricultural biowastes [7,8]. The management of biomass waste is a growing challenge in developing countries due to its increasing generation and the lack of feasible and efficient approaches to provide proper use of these resources [9]. Their conversion into value-added products is

Starch Cryogels Polymerization
Once the cryogels were obtained, PEDOT was synthesized via oxidative chemical polymerization of EDOT onto the starch scaffold, adapted from previous work [14]. The starch cryogel sample was placed in an alcoholic solution containing isopropanol (IPA) and iron(III) p-toluenesulfonate hexahydrate 0.3 M. The effect of the oxidizing agent on the synthesis of PEDOT was evaluated by soaking the starch cryogel in an iron(III) solution at different times (0. 5, 8, 24, and 48 h). Then, the samples were rinsed 5 times using IPA. Cryogels samples were immersed for 48 h in an IPA solution containing EDOT (2 M), resulting in a dark blue composite. The samples were then washed with IPA until the solution was colorless. The resulting scaffold was dried for 24 h in a vacuum oven (ADP 200C, Yamato-Scientific, Tokyo, Japan) at 45 • C and 85 kPa.

Physical Characterization of the Porous Materials during the Functionalization Process
Skeletal density of polysaccharide-based cryogels (ρ skel ) was determined using a nitrogen pycnometer (Ultrapyc 5000, Anton Paar, Graz, Austria) set at room temperature and 15 psi. A total of 10 replicates were used for each measurement. The bulk density (ρ bulk ) of the cylindrical cryogels was calculated by weighing and measuring the dimensions of each cryogel before functionalization and after soak times in iron(III) and EDOT polymerization. Equations (1) and (2) were used to calculate cryogels' overall percentage porosity (ε) and total pore volume (Vp), respectively, according to a previous report [42].
The specific surface area (A BET ) of the polymerized cryogels at different soak times in iron(III) solution was determined by applying nitrogen physisorption using the BET (Brunauer-Emmett-Teller) method in an adsorption analyzer (Anton Paar, Autosorb iQ, Graz, Austria). The characterization of the specific pore volume (Vp) and the mean of pore diameter (Dp) were estimated using the BJH (Barrett-Joyner-Halenda) method. Finally, micrographs of the polymerized cryogels were recorded via scanning electron microscopy (JSM-IT500 InTouch Scope; JEOL, Tokyo, Japan).

Analysis of the Composition of the PEDOT Polymerized in Porous Materials after Different Soak Times in Iron(III)
The composition of the cryogels was analyzed using a Fourier-transformed infrared spectroscopy (FTIR) spectrometer (Nicolet 380, Thermo Scientific, Madison, WI, USA) controlled with the OMNIC v9.3.30 software. For the analysis, PEDOT-polymerized starch cryogels were ground using a mortar and mixed with potassium bromide (KBr). The obtained powder was pressed using a Specac Atlas™ manual 15T hydraulic press (Specac Ltd., Orpington, UK) to obtain a pellet. Measurements of the acquired pellets were made in transmittance mode in the 4000-400 cm −1 spectral range using 64 scans with a resolution of 4 cm −1 .
Thermogravimetric analysis (TGA) was carried out for all cryogel samples after iron(III) immersion and polymerization in an SDT Q600 from TA Instruments (New Castle, DE, USA). A nitrogen atmosphere (100 mL·min −1 ) was maintained during the analysis with a scan rate of 10 • C·min −1 from 25 • C to 800 • C in alumina cups (110 µL) (TA Instruments, New Castle, DE, USA). Finally, the atmosphere was changed to synthetic air at a flow of 100.00 mL·min −1 .
The crystal structure of the inorganic components was studied with the X-ray diffractometer (XRD) (Empyrean, Malvern-PANalytical, Malvern, UK). The bulk material was packed on a sample holder and measured in the range of (2θ) 5 • -55 • with a cobalt (Co) anode and an Fe filter using a GaliPix detector with a 17.8 mm anti-scatter slit. The operating conditions including step size, voltage, and current were adjusted to 0.007 • , 40 kV, and 40 mA, respectively. Finally, the 2θ data was recalculated from Cobalt-K α to Copper-K α for analysis, the background was removed, and each diffractogram was normalized by area.
The total organic carbon (TOC) for the porous composites was measured using a CHNS analyzer (Vario TOC cube, Elementar, Langenselbold, Germany) using ca. 50 mg of sample. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analysis for the St/iron(III)/PEDOT cryogels after different soak times in iron(III) solution were analyzed using an energy dispersive X-ray spectrometer detector (EDS) (JSM-IT500, JEOL, Tokyo, Japan) and tabletop SEM (CUBE II, Emcrafts, Gwangju, Republic of Korea) with EDS Xplore Compact, Oxford, UK), applying a voltage of 30 kV. Image analysis was performed with the SMILE VIEW™ Lab software 2.1 (JEOL Ltd., Tokyo, Japan).

Effect of the Starch-Porous Material Iron(III) Soak Time on the Electrical Properties of the PEDOT Polymerized Cryogels
The electrical properties of the conductive material were evaluated via electrical impedance spectroscopy on a potentiostat instrument (AUTOLAB, PGSTAT-302 model, Utrecht, The Netherlands). The analysis was carried out in a system of two electrodes made of copper strips; the conductive material was located between the 2 copper electrodes placed in a compression load of ca. 35 mg. The resistivity of each probe was obtained from the current vs. voltage curves corrected by the area and length. The impedance analysis was performed at 50 mV in a frequency range of 0.1 to 10 5 Hertz (Hz).

Starch Extraction from Biomass Waste and Its Structuration
The obtained starch from the discarded potato matter was in the range of 7.82 ± 0.10%, in agreement with those values reported in the literature [43]. The extracted starch was used to generate a starch-based cryogel from a retrogradation of an aqueous solution and freeze-drying process (some properties are shown in the Figure S1). The obtained porous materials presented similar physical properties to those previously fabricated from commercial starch [21] (see Figure 1), exhibiting a novel strategy to process biomass waste into value-added products.
Utrecht, The Netherlands). The analysis was carried out in a system of two electrodes made of copper strips; the conductive material was located between the 2 copper electrodes placed in a compression load of ca. 35 mg. The resistivity of each probe was obtained from the current vs. voltage curves corrected by the area and length. The impedance analysis was performed at 50 mV in a frequency range of 0.1 to 10 5 Hertz (Hz).

Starch Extraction from Biomass Waste and Its Structuration
The obtained starch from the discarded potato matter was in the range of 7.82 ± 0.10%, in agreement with those values reported in the literature [43]. The extracted starch was used to generate a starch-based cryogel from a retrogradation of an aqueous solution and freeze-drying process (some properties are shown in the Figure S1). The obtained porous materials presented similar physical properties to those previously fabricated from commercial starch [21] (see Figure 1), exhibiting a novel strategy to process biomass waste into value-added products.

Characterization of the Starch Template, Iron(III)/Starch, and Conductive Composite Cryogels
The chemical composition of the composites at different soaking times was evaluated via infrared, thermogravimetry, and elemental composition. The obtained data confirmed starch-iron interaction and its effect on the conductive polymer deposition in the porous structure. Starch cryogels were poured into the IPA-iron(III) solution, and immediately, the coloration of the samples changed from white to dark yellow. This is attributed to the iron content in the solution, supposing that free ferric ions in the IPA-iron(III) solution bonded to the polysaccharide structure in accordance with previous reports [44].
The modification of the starch cryogel sample caused by the iron ions was monitored via TGA analysis. As is shown in Figure 2, the degradation mechanism of the cryogel samples was modified by the presence of iron ions. At lower soaking times (0.5 h), the material degrades, following a similar trend to that of the single template. However, longer soaking times show a decrease in the starch thermal stability, and no significant changes were observed after 8 h [45].

Characterization of the Starch Template, Iron(III)/Starch, and Conductive Composite Cryogels
The chemical composition of the composites at different soaking times was evaluated via infrared, thermogravimetry, and elemental composition. The obtained data confirmed starch-iron interaction and its effect on the conductive polymer deposition in the porous structure. Starch cryogels were poured into the IPA-iron(III) solution, and immediately, the coloration of the samples changed from white to dark yellow. This is attributed to the iron content in the solution, supposing that free ferric ions in the IPA-iron(III) solution bonded to the polysaccharide structure in accordance with previous reports [44].
The modification of the starch cryogel sample caused by the iron ions was monitored via TGA analysis. As is shown in Figure 2, the degradation mechanism of the cryogel samples was modified by the presence of iron ions. At lower soaking times (0.5 h), the material degrades, following a similar trend to that of the single template. However, longer soaking times show a decrease in the starch thermal stability, and no significant changes were observed after 8 h [45].  FTIR spectra of starch cryogels and PEDOT-polymerized starch cryogels after IPAiron(III) soaking are shown in Figure 3. The main characteristic absorption peaks of the polymerized samples in the region of 3000-3500 cm −1 show a shape change and shifting in the functionalized samples (Figure 2b,c), indicating a strong interaction between the polysaccharide structure and iron(III) ions [14,37,44]. Specifically, the absorption peaks at wavelengths around 3300 cm −1 could be attributed to the O-H stretching vibrations of structural starch OH. Signal changes between samples could imply the participation of starch hydroxyl groups in the complexion reaction [44,46]. Additionally, the signal at 2900 cm −1 corresponding to the C-H stretching vibration presented slight intensity differences among samples. The textural properties of the resulting cryogels for each synthesis stage are shown in Table S1 (see in Supplementary Materials). All samples presented low bulk densities ( ) with differences among St, St/iron(III), and St/iron(III)/PEDOT materials. These differences are attributable to the presence of iron(III) and PEDOT, as well as the reduction of empty spaces in the cryogels. The results agreed with previous reports using a similar concentration of corn starch [51] and for PEDOT three-dimensional structures [26].
Differences were determined between skeletal density values. St and St/ iron(III) cryogels showed skeletal densities ( skel) values in the range of native starch (1.50 g·cm −3 ) [16]. The pycnometer results showed that the St/iron(III)/PEDOT skeletal density decreased significantly, reaching similar values to those previously reported for PEDOT cryogels [26,52]. Porosity was determined using Equation (1), which is directly related to the geometrical scaffold density. The obtained values were higher than 85% for all samples.
We observed that the cryogel macroporous structure endured the polymerization process through all preparation stages without an evident collapse, as shown in Figure 4, even at long soak times. Regarding the microstructure, the nitrogen absorption data indicated low specific surface areas and micropore size for all the samples, which is common in samples processed in freeze-drying conditions [31]. Samples tended to show lower surface areas and a slight decrease in the micropore size at longer soak times. The physical properties of the obtained porous materials are summarized in Table S2 (see in Supplementary Materials). The signal at 1640 cm −1 is indicative of the stretching vibration absorption peak of C=O [37,46]. Shifting of this signal when the conductive polymer is deposited on the template may be due to changes in the starch/PEDOT interaction (see Figure 3 inset). The main bands found in the FTIR spectra at 1125, 1078, 1039, and near the region of 1024 cm −1 were attributed to coupled valent vibrations of the C-O and C-C bonds and deformational vibrations of the C-C-H and C-O-H bonds [46].
The band at 1153 cm −1 is identified for the starch template, and it was assigned to vibrations of the C-O-C group on the glycosidic bond; this signal shifted to 1125 cm −1 in polymerized samples, possibly related to the polysaccharide-iron interaction [47].
The signals at 860 and 692 cm −1 are identified in the starch/iron template and the polymerized sample (Figure 3b,c). Those bands are absent in the starch template spectrum. The same result was obtained previously by some researchers who mentioned that these signals correspond to the β-FeOOH group, confirming the formation of the polysaccharideiron(III) complexes [47][48][49].
Finally, the signal at 1438 cm −1 is associated with thiophene symmetric C=C stretching and its oxidation state, confirming the polymerization of EDOT in all samples [50] (see Figure S2, in Supplementary Materials).
The textural properties of the resulting cryogels for each synthesis stage are shown in Table S1 (see in Supplementary Materials). All samples presented low bulk densities (ρ bulk ) with differences among St, St/iron(III), and St/iron(III)/PEDOT materials. These differences are attributable to the presence of iron(III) and PEDOT, as well as the reduction of empty spaces in the cryogels. The results agreed with previous reports using a similar concentration of corn starch [51] and for PEDOT three-dimensional structures [26].
Differences were determined between skeletal density values. St and St/ iron(III) cryogels showed skeletal densities (ρ skel ) values in the range of native starch (1.50 g·cm −3 ) [16]. The pycnometer results showed that the St/iron(III)/PEDOT skeletal density decreased significantly, reaching similar values to those previously reported for PEDOT cryogels [26,52]. Porosity was determined using Equation (1), which is directly related to the geometrical scaffold density. The obtained values were higher than 85% for all samples.
We observed that the cryogel macroporous structure endured the polymerization process through all preparation stages without an evident collapse, as shown in Figure 4, even at long soak times. Regarding the microstructure, the nitrogen absorption data indicated low specific surface areas and micropore size for all the samples, which is common in samples processed in freeze-drying conditions [31]. Samples tended to show lower surface areas and a slight decrease in the micropore size at longer soak times. The physical properties of the obtained porous materials are summarized in  TGA data demonstrated that no significant difference in ashes was observed after an 8 h soaking time, and a similar amount of conductive polymer was observed in the 24 h and 48 h of soak time samples (see Table 1). Consequently, EDS analysis confirmed that iron is present in the samples, and sulfur (associated with the PEDOT polymer) ( Figure 5) was deposited onto the template (see overlayed images). TGA data demonstrated that no significant difference in ashes was observed after an 8 h soaking time, and a similar amount of conductive polymer was observed in the 24 h and 48 h of soak time samples (see Table 1). Consequently, EDS analysis confirmed that iron is present in the samples, and sulfur (associated with the PEDOT polymer) ( Figure 5) was deposited onto the template (see overlayed images).  XRD diffraction ( Figure 6) analysis showed patterns for the starch template in all the samples [53]. After the PEDOT deposition, a peak profile associated with the conductive polymer is observed, showing some shifting due to molecular structural rearrangements caused by changes in its doing level [35,54]. Additionally, small, broad, and overlayed peaks are observed in the XRD data; they could be related to amorphous iron compounds. Similar results were reported during the synthesis of PEDOT using FeCl 3 as an oxidant in the absence of a template [34]. Therefore, considering the iron and PEDOT tendency to saturate the template after an 8 h soak time, our FTIR and XDR results may indicate that iron ions differ in their coordination interactions with the starch template along the soak time. XRD diffraction ( Figure 6) analysis showed patterns for the starch template in all the samples [53]. After the PEDOT deposition, a peak profile associated with the conductive polymer is observed, showing some shifting due to molecular structural rearrangements caused by changes in its doing level [35,54]. Additionally, small, broad, and overlayed peaks are observed in the XRD data; they could be related to amorphous iron compounds. Similar results were reported during the synthesis of PEDOT using FeCl3 as an oxidant in the absence of a template [34]. Therefore, considering the iron and PEDOT tendency to saturate the template after an 8 h soak time, our FTIR and XDR results may indicate that iron ions differ in their coordination interactions with the starch template along the soak time.  Elemental analysis data (Table S3, see in Supplementary Materials) showed that the ratio of sulfur in the sample decreased after 8 h of soaking. The difference between TOC and TGA results suggests that a longer soaking time decreases the amount of counter ion (i.e., p-toluenesulfonate ion) in the matrix. The counter ion is allowed to freely diffuse to the solvent since a longer interaction time results in an effective stabilization via coordination of the iron species into the starch template [38].
Electrical impedance spectroscopy characterization (EIS) of the porous composites was performed using a previous model [21], which included the mass transfer phenomena, the effects of the distribution of energy states, the porosity of the materials, and the electrical conductivity. The impedance magnitude and phase of porous composites at different soaking times are shown in Figure 7. The 0.5 h soaking time sample shows a low impedance and a resistive behavior, which is probably due to the existence of free ions (e.g., iron(III) ions) [55,56]. A longer soak time unveiled higher impedance values and more complex electrical behavior.
was performed using a previous model [21], which included the mass transfer phenomena, the effects of the distribution of energy states, the porosity of the materials, and the electrical conductivity. The impedance magnitude and phase of porous composites at different soaking times are shown in Figure 7. The 0.5 h soaking time sample shows a low impedance and a resistive behavior, which is probably due to the existence of free ions (e.g., iron(III) ions) [55,56]. A longer soak time unveiled higher impedance values and more complex electrical behavior. The real impedance (Z′) and imaginary impedance (Z″) (Figure 8) confirmed the effect of the starch/iron interaction time in the electron transfer mechanism on the porous conductive samples (Figure 7). At a short iron starch interaction time (0.5 h), a dominant resistive behavior was identified. Zhang et. al. have reported that the concentration of iron(III) ions in a polymeric network decreases the electrical resistance [56]. Longer soaking times unveiled a more complex electrical mechanism, including contact resistance, the impedance linked to the intraparticle porous material, and the impedance related to the mass transfer region as well as a second semicircle, suggesting a complementary electrical mechanism due to the iron complexation by the starch. In general, the data describe a system containing two layers or two regions with different electrical properties [57]. The real impedance (Z ) and imaginary impedance (Z") ( Figure 8) confirmed the effect of the starch/iron interaction time in the electron transfer mechanism on the porous conductive samples (Figure 7). At a short iron starch interaction time (0.5 h), a dominant resistive behavior was identified. Zhang et. al. have reported that the concentration of iron(III) ions in a polymeric network decreases the electrical resistance [56]. Longer soaking times unveiled a more complex electrical mechanism, including contact resistance, the impedance linked to the intraparticle porous material, and the impedance related to the mass transfer region as well as a second semicircle, suggesting a complementary electrical mechanism due to the iron complexation by the starch. In general, the data describe a system containing two layers or two regions with different electrical properties [57].
The impedance data fitting was performed using Autolab NOVA 2.1.6. based on a previous model [21]. The model consists of an equivalent electrical circuit comprising a series connection of two parallel circuits. The parameters obtained from the impedance modeling are reported in Table S4 (see in Supplementary Materials). The 0.5 and 8 h samples were not modeled using the complete model because these samples showed a single semicircle. Longer soaking times unveiled higher impedance values and more complex electrical behavior because of the lower availability of free iron ions by complexation for the biopolymer matrix along with the diffusion of counter ions during the sample preparation. The fitting data confirmed that a longer soaking time led to a higher capacitance for the first and second semicircles, which is expected for conductive polymer deposition [21].
Overall, biomass-waste starch was a suitable raw material to structure a porous template. The porous structure withstands the fabrication process with only slight differences in microporosity. The chemical composition at different soaking times was studied via infrared, thermogravimetry, and elemental composition, confirming that a longer soak time altered the starch-iron ions interaction. Additionally, the results evidenced the presence of PEDOT and counter ions through the synthesis process, and they influenced the electrical behavior of the obtained composites. The impedance data fitting was performed using Autolab NOVA 2.1.6. based on a previous model [21]. The model consists of an equivalent electrical circuit comprising a series connection of two parallel circuits. The parameters obtained from the impedance modeling are reported in Table S4 (see in Supplementary Materials). The 0.5 and 8 h samples were not modeled using the complete model because these samples showed a single

Conclusions
Our work gives insights into the methodology to functionalize starch obtained from biomass waste using chemical polymerization of poly(3,4-ethylenedioxythiophene) directly on the substrate. Thermal and spectrometric characterization of those composites allow us to detect and quantify the iron-starch interaction and the amount of oxidizing agent during the soaking time. Moreover, we have confirmed using electronic microscopy and nitrogen physisorption that the porous template endured the polymerization process through all preparation stages without an evident collapse on the macrostructure, slightly affecting the microstructure due to the PEDOT deposition on the template surface. Longer soaking times (i.e., 48 h) unveiled, via impedance spectroscopy, a more complex electrical behavior, which is of great interest to a wide audience, including electronic, environmental, and biological applications. Finally, the conversion of potato waste into a conductive porous composite is an example of the potential preparation of innovative materials that balance progress and sustainability in a circular economy framework.  Table S1: Physical properties of the porous obtained materials at each stage of synthesis; Table S2: Physisorption Nitrogen physisorption data of cryogel template and conductive porous composites at different soaking times; Table S3: Elemental composition of porous structures synthesized with PEDOT at different immersion times in iron(III) solution; Table S4: Fitting of electrochemical impedance spectroscopy data using the R(RQ)(RQ) circuit model.